Operation of turbine engines is well known. Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produces two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.
In the above circumstances it will be appreciated that there is a general air flow through the engine 10. It will also be understood that turbine engines 10 can be utilised in order to provide electrical power through a generator typically driven by a coupling to the compressor/turbine spool. In short through that coupling which will generally also incorporate a gear box, an electrical generator is driven in order to provide electrical power for an aircraft.
Clearly, there may be differences in the demand for electrical power and the demand for engine thrust. Traditionally, this divergence has been accommodated through dumping compressor airflow to the engine bypass and so does not necessarily result in a reduction in thrust by the engine. It will be understood that in addition to considering the thrust requirements as well as electrical power demand, it is also necessary to ensure that compressor stability is maintained. As indicated previously, traditionally compressor stability has been maintained by bleeding air into the bypass but with thrust recovery. Unfortunately, in situations where thrust recovery is not required, for example during descent or ground idle, the excess thrust can result in a longer descent than desirable or excess wheel brake wear on the ground.
In such circumstances it will be desirable to maintain compressor stability under reduced thrust load whilst still maintaining sufficient work capacity in order to drive such auxiliary machines as electrical power generators.